Method of coating heater coils



June 17, 1969 J. THEooosdPouLos ET AL 3,450,565

METHOD OF COATING HEATERCOILS I Filed Dec. 18 1964 JAMES THEODOSOPOULOS STANLEY R. MACKAY IN VENTORS In? ATTORN United States Patent 3,450,565 METHOD OF COATING HEATER COILS James Theodosopoulos, Ipswich, Mass., and Stanley Mackay, Exeter, N.H., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Dec. 18, 1964, Ser. No. 419,440 Int. Cl. B44d 1/18; H0111 1/02 US. Cl. 117212 1 Claim heater wire from each other and from the cathode in which it will be disposed. The aluminum oxide coating is normally white, however, it is advantageous to have a gray or black colored coating on the heater. The higher emissivity of the dark coating results in better heat transfer to the cathode. Therefore, for the necessary cathode temperature and adequate electron emission, a dark coated heater would operate at a lower temperature than a white coated heater. The advantages resulting from this lower heater temperature are longer life, better current stability of the heater and lower heater-cathode electrical leakage.

Heater coils are an essential element in most vacuum tubes. These coils are placed inside thin walled, nickel sleeves which are coated on the outside with carbonates of barium, calcium, and strontium. After a coil is inserted in a cathode sleeve, the assembly is heated in a vacuum to reduce the carbonate to their corresponding oxides. When properly activated, electrons will be emitted and the coated sleeve will act as a cathode. Connection of the coil to a power supply will furnish heat to the sleeve to activate the oxides.

Conventional processes for obtaining a dark coating on the heaters are by direct use of a coating suspension that incorporates insoluble metallic oxide powders, such as, chromium or titanium, which will impart a dark color to the coating, after the coated heater has been fired. The major disadvantage to this process is that these metallic oxides have poorer electrical insulation characteristics at normal heater operating temperatures than the white aluminum oxide coating, and therefore, have higher heater-cathode electrical leakages, which results in a poorer quality of short-life tube.

Another method of obtaining a dark coated heater is to directly deposit a dark coating over a conventional white coated heater. In this process, the heater is conventionally coated with the white aluminum oxide. It may then, although not necessarily, be fired at a high enough temperature to sinter the coating. The sintered, coated coil is then electrophoretically or dip coated in a suspension of fine tungsten powder to incorporate the metal and produce a dark coating.

According to our invention, the heater coil is coated with aluminum oxide such as disclosed in the US. patent of Dolan, No. 3,049,482. Following the manufacture, the coated coil is dipped in a solution of a soluble high tungsten containing compound. After dipping, the coated coil is sintered at an elevated temperature in a reducing atmosphere and the ionic tungsten is reduced to a tungsten metal, thereby imparting the dark coloration.

Accordingly, the primary object of our invention is darkening an insulating coating on heater coils.

Another object of our invention is increasing the emissivity of the coating on the heater coil, hence, resulting in better heat transfer without decreasing the insulating characters.

The many other objects, features and advantages of 3,450,565 Patented June 17, 19 69 the present invention will become manifest to those conversant with the art upon reading the following specification and drawings wherein the specific embodiments of a method of placing a dark overcoat on the insulating coating are shown and described.

Of these figures:

FIGURE 1 is an elevational view of a reverse helix heater coil coated with alumina and tungsten.

FIGURE 2 is an elevational view of a hair-pin shaped heater coil coated with aluminum and tungsten.

FIGURE 3 is a cross-sectional view taken along the line 3-3 of FIGURES l and 2.

A tungsten heater-coil having the general shape of a reverse helix of a hair pin is electrophoreticallycoated with a white insulating oxide such as aluminum according to well known and conventional techniques in the art. Following the coating, the cathode is heated and dried. When dry, it is dipped to a predetermined height in a solution of a soluble tungsten compound. Preferably, the entire white coating is not covered and a small gap is left on the aluminum oxide between the legs of the coil and the tungsten coating. Such gaps minimize heater cathode leakage. Preferably, the tungsten coating solution is saturated with the compound but contains at least greater than about 0.25 gram atoms of tungsten per liter of solvent. After coating with tungsten containing solution, the heater is fired at a high temperature, usually greater than 1300" C., in a reducing atmosphere such as hydrogen. The firing and the reducing atmosphere reduces the ionic tungsten to the metal and darkens the coating.

This process is highly advantageous because deposition through solution is easy to control. Since the tungsten is in ionic form in the solvent, vigorous agitation is not required to maintain a suspension. The heater coils which are produced are more uniform in color and therefore are more uniform in electrical characteristics. Heatercathode leakage is lower and therefore better quality, longer life tubes can be attained.

As sources of the tungsten compound, we can use any heteropoly tungstic acid such as phosphotungstic acid, aluminotungstic acid or preferably tungstosilicic acid. These sources of tungsten can be suspended in any suitable polar solvent such as methyl or ethyl alcohol, water, acetone, etc. Any heteropoly tungstic acid which has adequate solubility in polar solvents can be used provided that the hetero atom, that is, as described above, the silicon, the aluminum or the phosphorus, would sublime completely during the firing process or be rendered sufficiently inert to maintain good electrical insulation in the quantity present. For example, such non-conductive, hetero molecules would be aluminum oxide or silicon dioxide. The phosphorus in phosphorus based materials would sublime.

Referring now to FIGURE 1 of the drawing, the heater coil 1 is coated with aluminum oxide and covered with tungsten metal particles. Each coil is made of tungsten metal and coated with aluminum oxide 3. When coated, the coil 1 is dipped in the tungsten-ion containing solution and a darkened coating will appear after firing in a reducing atmosphere. A small gap 5 is left between the legs 4'of the coil and the tungsten impregnates aluminum oxide coating to prevent leakage, The shape of the coil is immaterial to the invention and many other designs are possible such as shown in FIGURE 2. A hair-pin shape having a similar aluminum oxide coating 6 upon the coil 7 may be successfully darkened with an overcoat 8' of tungsten. A small gap is left between the tungsten overcoat 8 and the coil 7 to prevent leakage.

As shown in FIGURE 3, the tungsten wire 9 has an aluminum oxide layer 10 coated thereupon. The tungsten overcoat 11 impregnates only partly through the coating 10 to the wire 9, preferably.

As specific examples of our invention we cite the following: a tungsten heater coil having a hair-pin shape is electrophoretically coated with white aluminum oxide and leaving the legs uncoated. The coated heater is dried and then dipped into a solution containing 100 gms. of tungstosilicic acid in 100 ml. of methyl alcohol. The heater is then fired at 1550 C. in a hydrogen atmosphere and the coating of metallic tungsten appears on the outside.

It is apparent that modifications and changes may be made within the spirit and scope of the instant invention. 10

It is our intention, however, to be limited only by the appended claim.

We claim as our invention:

1. In the process of preparing heater coils coated with an insulating oxide but having uncoated ends, the improvement comprising:

preparing a solution of at least one member selected from the heteropoly group consisting of phosphotungstic acid, aluminotungstic acid and tungstosilicic acid;

References Cited UNITED STATES PATENTS 7/1965 Hassett 313340 OTHER REFERENCES German Auslegeschrift 1,090,774, Schmidt et al.

WILLIAM L. JARVIS, Primary Examiner. 

1. IN THE PROCESS OF PREPARING HEATER COILS COATED WITH AN INSULATING OXIDE BUT HAVING UNCOATED ENDS, THE IMPROVEMENT COMPRISING: PREPARING A SOLUTION OF AT LEAST ONE MEMBER SELECTED FROM THE HETEROPOLY GROUP CONSISTING OF PHOSPHOTUNGSTIC ACID, ALUMINOTUNGSTIC ACID AND TUNGSTOSILICIC ACID; DIPPING SAID COATED HEATER INTO SAID SOLUTION TO A DEPTH SUFFICIENT TO LEAVE AN UNDIPPED GAP OF INSULATING OXIDE COATING AT THE ENDS THEREOF; AND FIRING SAID DIPPED COIL IN A REDUCING ATMOSPHERE UNDER CONDITIONS SUFFICIENT TO REDUCE SAID MEMBER TO TUNGSTEN METAL AND THEREBY DARKENING THE SURFACE OF SAID INSULATING COATING. 